As a photoelectric conversion device used for a solar cell that converts the energy in sunlight into electrical energy, there is a known thin-film silicon photoelectric conversion device having a photoelectric conversion layer fabricated by forming thin films of a p-type silicon semiconductor (p-layer), an i-type silicon semiconductor (i-layer), and an n-type silicon semiconductor (n-layer) by using a plasma CVD method or the like.
Thin-film silicon solar cells have the advantages that the area thereof can be easily increased, and only a small amount of material is used because the film thickness thereof is about one hundredth of that of crystalline solar cells. Therefore, thin-film silicon solar cells can be fabricated at lower cost than crystalline solar cells. However, thin-film silicon solar cells have the disadvantage that the conversion efficiency thereof is lower than that of crystalline solar cells. In this technical field, an improvement in the conversion efficiency is a key issue, and a tandem solar cell having a photoelectric conversion layer in which two power generation cell layers are laminated has been proposed.
In a tandem solar cell, in order to suppress dopant interdiffusion between a first power generation cell layer and a second power generation cell layer and to adjust the light distribution, an intermediate contact layer formed of a transparent conductive film is inserted therebetween. For the intermediate contact layer, Ga-doped ZnO (GZO) is used in general. GZO has a refractive index of 2.0, which is lower than Si, and is a material having excellent plasma resistance and excellent transparency.
However, since GZO has low resistivity, when it is used for an integrated solar cell module, it causes leakage current in a cell-connected portion, thereby reducing the open circuit voltage and FF. In order to prevent the leakage current, solutions have been proposed, such as adding a laser-processed portion to the structure of the connected portion. However, adding a new processed portion causes the problems of decreased effective area and increased cost.
An improvement in FF has been attempted by controlling the conductivity of GZO to increase the resistance of GZO. As described in PLT1, the conductivity of GZO can be controlled by reducing the amount of dopant or by adjusting the amount of oxygen supplied at the time of film formation to promote oxidation of GZO.